TRANSIENT NATURAL-CONVECTION HEAT AND MASS-TRANSFER OVER A VERTICAL PLATE OF FINITE HEIGHT

Abstract

A detailed numerical analysis has been carried out to study transient natural convection in a binary mixture adjacent to a vertical plate of finite height with the simultaneous presence of temperature and concentration gradients. The full elliptic Navier-Stokes equations and the energy and species diffusion equations were solved together by the projection method with the convective terms treated by the QUICK scheme. Computations were performed for a mixture with Pr = 0.7, Sc = 3.5, and thermal Grashof number fixed at 10(6). Both the augmenting and opposing flows were investigated with the buoyancy ratio varied from - 5 to 5 and the concentration ratio, an interface velocity parameter, from 10 to 1000. Results are presented to illustrate, in particular, the effects of combined thermal and solutal buoyancy forces on the temporal evolution of the flow pattern and the associated heat and mass transfer. Representative transient velocity, temperature, and concentration profiles are presented, along with the transient local and average Nusselt and Sherwood numbers. In the initial transient, the interface velocity at the plate induced by the concentration gradient dominates the flow patterns. After the initial period, the recirculating flows generated by the two buoyancy forces in the fluid are prevalent. In opposing flows, countercurrent flows may result at a high buoyancy ratio.